Optics of keratoprostheses

Purpose. To determine the optimal geometry for keratoprostheses. Methods. A number of biopolymers were considered, among them, various plastics such as pMMA or pHEMA and various polysiloxanes. Their refractive index and dispersion were measured using an Abbe refractometer (Zeiss, Germany) and the values were incorporated in the framework of an eye model derived from the Gullstrand's eye model. Ray-tracing methods were used to determine the optimal parameters of the keratoprosthesis as a function of the eye parameters. The field of view was then calculated using paraxial approximations. Results. Flat designs, i.e. designs where the length of the keratoprosthesis is smaller than its diameter, are advisable because the field of view is enlarged and decentration problems are minimized. If the material used has a low refractive index, optical aberrations should be corrected with aspherical surfaces. Conclusions. New flexible biopolymers with a low refractive index such as polytrifluorosiloxane, are potential candidates for keratoprosthesis. They are mimicking the actual cornea, allowing a design as close as possible to the physiological conditions, while their flexibility may enable IOP measurements.